loanCalculator.js

/* This function extracts user input from the Loan Calculator input form and uses it to set the variables of loanAmount, repayPeriod and protection*/

function getInput()
{
    document.getElementById('error').style.display = 'none';
    var loanAmount = document.getElementById('loan_amount').value;
    
    /*Check whether user input for loan amount is valid*/
    if (isNaN(loanAmount) == true || loanAmount<1000 || loanAmount>500000)
    {
        document.getElementById('error').style.display = 'block';
        document.getElementById('loan_amount').value = "";
        return false;
    }
    
    var selectedPeriod = document.getElementById("repay_period");
    var selectedPeriod = selectedPeriod.options[selectedPeriod.selectedIndex].text;
    var protection = false;
    var repayPeriod = 0; //initialise repayPeriod variable
    
    /*Convert the repay period selected from the drop-down list into the number of months in the repay period*/
    if (selectedPeriod == "6 months")
    {
        repayPeriod = 6; //Deal with special case of 6 months
    }
    else 
    {
        repayPeriod = parseInt(selectedPeriod.split(/\s+/)[0]) * 12; //extract number from repay period text and multiply by 12 to get months
    }
    
    /*Check if user has selected payment protection*/
    if (document.forms[0].protect.checked)
    {
        protection = true;
    }
    
    calculateRepayment(loanAmount, repayPeriod, protection);
}

/*This function calls functions to calculate interest, payment protection and monthly payments and outputs to form*/

function calculateRepayment(loanAmount, repayPeriod, protection)
{
    var interestRate = calculateInterestRate(loanAmount);
    document.getElementById('rate').value =interestRate + "%";
    
    var totalInterest = calculateInterest(loanAmount, repayPeriod, interestRate);
    document.getElementById('total_interest').value = totalInterest;
}

/*This function uses the loan amount to determine the interest rate*/

function calculateInterestRate(loanAmount)
{
    var baseRate = 4.25;
    var rate = 0;
    
    if (loanAmount <= 10000)
    {
        rate = baseRate + 5.5;
    }
    else if (loanAmount <= 50000)
    {
        rate = baseRate + 4.5;
    }
    else if (loanAmount <= 100000)
    {
        rate = baseRate + 3.5;
    }
    else 
    {
        rate = baseRate + 2.5;
    }
    
    return rate;
}


function calculateInterest(loanAmount, repayPeriod, interestRate)
{
    var interestYears = repayPeriod / 12;
    var rate = interestRate / 100;
    return loanAmount * rate * interestYears;
}